Is it permissible for a wife to keep a portion of her husband's money that exceeds her needs without his knowledge, intending to return it to him upon his return?

If the husband sends you money as an obligatory maintenance payment, then any surplus from it is your property, and you may dispose of it without his permission. However, if he sends it for you to spend on yourself and the household as an agent, then the surplus belongs to him, and you may not dispose of it except with his permission.